Used as orally administrable antibacterial agents, oxazolidinone compounds are not products of fermentation, but artificially synthesized ones, and various structures of their derivatives are known. For instance, 3-phenyl-2-oxazolidinone derivatives having one or two substituents are stated in U.S. Pat. Nos. 4,948,801, 4,461,773, 4,340,606, 4,476,136, 4,250,318 and 4,128,654. 3-[(Monosubstituted) phenyl]-2-oxazolidinone derivatives of Formula 2 are disclosed in EP 0312000, J. Med. Chem. 32, 1673 (1989), J. Med. Chem. 33, 2569 (1990), Tetrahedron, 45, 123 (1989), etc.

Pharmacia & Upjohn developed oxazolidinone derivatives of Formulas 3 and 4 (WO 93/23384, WO 95/14684 and WO 95/07271). Having succeeded in gaining the approval of the Food and Drug Administration (FDA) of U.S.A., the oxazolidinone derivative of Formula 3, by the name of ‘Zyvox’, has came into the market. However, these conventional synthetic oxazolidinone compounds were found to suffer from the disadvantage of showing antibacterial activity against a narrow spectrum of bacteria, being toxic to humans, and being poor in therapeutic activity in vivo. Zyvox may be used restrictively as injection since the solubility of Zyvox against water is inadequate for use in injection, which is about 3 mg/ml.

Further, WO 93/09103 discloses derivatives of phenyl oxazolidinone, substituted with heterocyclics such as thiazole, indole, oxazole and quinole, as well as pyridine, at position 4 of the phenyl ring. However, these derivatives of oxazolidinone are known as providing insufficient medicinal effects because the heterocyclics bear simple substituents such as alkyl or amino groups.
In WO 01/94342, synthesizing derivatives of phenyl oxazolidinone, having with pyridine or derivatives of phenyl at position 4 of the phenyl ring was described. The compounds synthesized are potent in inhibitory activity against a broad spectrum of bacteria and are also superior antibiotic to Zyvox. However, the compounds are unable to be formulated as injection because solubility of the same is under 30 μg/ml.
Accordingly, the intensive and thorough research on oxazolidinone derivatives, conducted by the present inventors aiming to overcome the above problems encountered in prior arts, resulted in the finding oxazolidinone derivatives as well as prodrugs thereof, wherein the prodrugs are prepared by reacting amino acid or phosphate with the oxazolidinone derivatives having hydroxyl group. Further, salts of the oxazolidinone derivatives prodruged were easily synthesized by using amine group of amino acid of the same to synthesize organic acid or inorganic acid and by using a hydroxyl group of phosphate and one selected from sodium and calcium. The oxazolidinone derivatives have excellent effects on antibiotic activity and the solubility of the same is greatly enhanced.